Electrodeless discharge lamps are well known in the art and typically include an ionizable medium within a sealed envelope including at least one particular ionizable gas at a given pressure capable of emitting radiant energy when subjected to a radio frequency field. An electric field having a magnitude sufficient to initiate ionization of the ionizable medium to form a radiation emitting discharge is coupled to the medium. Simultaneously, a radio frequency (RF) magnetic field for maintaining ionization is coupled to the ionizable medium. If the various parameters of the lamp are properly selected, a high efficiency electrodeless fluorescent lamp is theoretically possible.
It is known to drive the ionizable medium of such lamps by means of a circuit which may include a crystal-controlled oscillator for generating an output signal at a given radio frequency, an RF amplifier responsive to the oscillator output signal, and an inductive output coil and a capacitor responsive to the output of the amplifier. The output coil is positioned in close physical proximity to the envelope for coupling to the ionizable medium the electric and magnetic fields.
Such electrodeless discharge lamps are often operated in an ISM band at a frequency of 13.56 MHz, because the Federal Communications Commission, as well as the rest of the world, permits such frequency to be used with great liberality. However, when operating at such a frequency, a number of problems are created.
The first problem is selecting an amplifier circuit which will operate efficiently. Converting input energy into output power in an efficient manner is essential if an electrodeless discharge lamp is to compete effectively with other types of lamps. A Class A amplifier is known to have a very low efficiency, generally less than 50%, rendering it unsuitable for the present application. A Class B amplifier has the potential of being about 78.5% efficient, but, in reality, generally runs significantly less than this, rendering it unsuitable. A Class C amplifier is very sensitive to various capacitances within the circuit so that a Class C circuit does not lend itself well to mass production. Furthermore, transistors having a rating two times the DC input supply are generally required and this can present significant problems.
A Class D amplifier not only has the potential of being 100% efficient because it functions as an on/off switch, but also requires transistor ratings of only 125% of the DC input supply. Moreover, a Class D amplifier is typically not dependent on device-related parameters. On the other hand, several factors suggest against the use of a Class D amplifier. First of all, at frequencies above 2 MHz it is difficult to generate fast-switching waveforms across the inevitable circuit capacitances and to keep the power dissipation low when switching times are not small compared to the RF period. Second, push-pull type Class D circuits are especially vulnerable because simultaneous conduction in the two transistors can cause catastrophic failure.
A Class E amplifier, like a Class D amplifier, has the potential of being 100% efficient because it also functions as an on/off switch and it too is not dependent on device-related parameters. However, a Class E amplifier has an even worse voltage potential than a Class C amplifier since the typical single semiconductor switch requires a rating of as much as four times the DC input supply due to the high voltages developed across the switching device during operation. Since the Class E amplifier is a switching amplifier, the power semiconductor must be able to be switched on and off at the required frequency of operation.
Operation at a frequency of, for example, 13.56 MHz requires high speed switching semiconductors with low input and output capacitances and low on-state resistance. This requirement for low capacitance and low on-state resistance is in direct conflict, from a device standpoint, with the requirement for a high breakdown voltage. While it is possible to reduce the breakdown voltage of the switching semiconductor by reducing the AC voltage at the input to the DC supply by means of, for example, a step-down transformer or an AC input capacitor, such alternatives add significant cost to the circuit. In instances where the operating circuit is contained within the base of an electrodeless lamp unit intended as a replacement for an incandescent lamp, space and/or weight requirements may prohibit such alternatives.
Another problem arising from operating at a high frequency, such as 13.56 MHz, is that the quantity of electromagnetic interference (EMI) and radio-frequency interference (RFI) produced could interfere with some other allocated frequencies. An RF amplifier operating at a fundamental frequency of 13.56 MHz will produce a fourth harmonic frequency at 54.24 MHz, a fifth harmonic frequency at 67.80 MHz, and harmonics at other multiples of the fundamental. Since the first two harmonics (27.12 MHz band 40.68 MHz) are also ISM bands, the main concern is with the outband noise. While almost all the RF energy is at 13.56 MHz, the amount of noise in the fourth and higher harmonics along with outband noise between the allowable ISM bands should be at a minimum.
U.S. Pat. No. 4,245,178, which issued to James W. H. Justice, describes a high-frequency electrodeless discharge device operated by RF energy which is generated by a single transistor oscillator operating in a Class E mode. Because of the relatively low operating frequency of 100 kHz, highly efficient switching transistors having the necessary high voltage rating are readily available. However, switching devices having a high voltage rating and capable of high switching efficiencies at higher frequencies, such as 13.56 MHz, are not readily available.
U.S. Pat. No. 4,048,541, which issued to Adams et al, describes a crystal controlled Class D oscillator circuit for illuminating an electrodeless fluorescent lamp at a frequency of approximately 13.56 MHz. The circuit includes an output coil (16) connected to transistors (50,52) which drive the coil in a non-symmetric, push-pull operation.
To overcome the difficulties mentioned above, the present invention proposes a circuit for operating an electrodeless discharge lamp in which the circuit contains an RF amplifier operating in a Class E mode and having a pair of switching devices connected in series. Moreover, the amount of EMI/RFI is significantly reduced by the use of a symmetrically-driven output coil.